1. Field of the Invention
The present invention relates a method of manufacturing a circuit board used for various switch devices, and more particularly, to a method of manufacturing a circuit board used for a slide-type switch device in which a movable contact slides on a fixed contact.
2. Description of the Related Art
In general, in a method of manufacturing a circuit board used for a slide-type switch device, first, a copper-clad insulating substrate formed by cladding one surface or both surfaces of an epoxy resin containing glass with copper is prepared, and the copper foil is etched to form fixed contact and lead conductor patterns. Then, a solder resist is coated on a specific region on the insulating substrate except for the fixed contact, and a nickel layer (a thickness of about 2 μm) and a silver layer (a thickness of about 5 μm) are sequentially formed on the fixed contact by plating. In this manufacturing method, when an oxide film remains on the surface of the copper foil during the etching process and the process of coating the solder resist, the adhesion of a resist or a solder resist used for etching is considerably reduced. Therefore, a process of polishing the surface of the copper foil with buff is additionally performed before the etching process and the solder resist coating process to remove the oxide film from the surface of the copper foil (for example, see Japanese Utility Model Application Laid-Open No. 6-77277).
In the circuit board manufactured through the above-mentioned processes, since the total sum of the thicknesses of the nickel layer (base layer) and the silver layer formed on the copper foil, which is a raw material forming the fixed contact, is about 7 μm, it is possible to planarize the silver layer, which is the uppermost layer of the fixed contact. Thus, this to lengthens the life span of the fixed contact and a movable contact sliding thereon. However, this structure has a problem in that the silver layer is sulfurized, which may cause the reliability of electrical connection to be lowered. Therefore, a switch device requiring a long life span and high reliability, for example, a switch device for a vehicle-mounted turn signal switch or steering switch that needs to be used hundreds of thousands of times is plated with gold, instead of silver. In this case, a gold layer is formed on the nickel layer, which is a base layer, with a predetermined thickness by plating. However, since gold is considerably more expensive than silver and has high malleability, the thickness of the gold layer is generally set to about 0.05 μm.
As described above, in the circuit board having the gold layer as the uppermost layer of the fixed contact, since the uppermost layer of the fixed contact is plated with gold, the reliability of electrical connection is improved, but a sufficiently long life span is not obtained. The inventors' study proved that a strip-shaped uneven portion occurs in the surface of the gold layer, and when the sliding direction of a movable contact is orthogonal to the direction in which the uneven portion extends, the abrasion loss of the fixed contact and the movable contact increases due to the uneven portion. The uneven portion occurs in the surface of the gold layer due to scars occurring in the surface of the copper foil during a polishing process using buff. That is, in a thick laminated structure of the nickel layer and the silver layer having a thickness of about 7 μm, since a thick nickel layer is formed on the uneven portion occurring due to the scars caused by the buffing process, little unevenness occurs in the surface of the uppermost layer. However, in a thin gold layer having a thickness of about 2 μm, unevenness occurs in the surface of the gold layer due to the scars occurring in the surface of the copper foil during the buffing process. This is because, regardless of the direction in which the surface extends, a plated layer tends to be substantially uniformly formed in a direction orthogonal to the surface and the gap between the surfaces of both sides of one concave portion becomes narrower when the plated layer is formed. In particular, when new buff having a sharp leading end is used, the depth of the scars caused by the buffing process increases, which causes the depth of an uneven portion occurring in the surface of the gold layer to increase. As a result, the abrasion of the fixed contact or the movable contact is accelerated.